1. Field of the Invention
The present invention relates to a spring steel and a method for production thereof, particularly to a spring steel suitable for fabricating high-strength coil springs for use in vehicles and general machinery.
2. Description of the Related Art
Today""s increasingly compact, high-performance vehicles and machines must be equipped with stronger springs. Particularly important to spring performance are fatigue resistance and permanent fatigue resistance. Coil springs are fabricated by either hot or cold coiling. Cold coiling is, however, generally adopted for springs requiring not only high strength but also small wire diameter, such as those used in vehicle engine valves. Recently, cold coiling is being used increasingly even for springs of large wire diameter such as suspension springs. The conventional practice in cold-coiled springs has been to use an oil-tempered wire made of a Sixe2x80x94Mn system or Sixe2x80x94Cr system spring steel such as set out in JIS G 4801. Moreover, as taught by, for example, JP-A-(unexamined published Japanese patent application)1-83644 or JP-A-2-57637, in order obtain still higher strength, the practice has been to use oil-tempered steel wire obtained by adding Mo, V and the like to the aforesaid base spring steel.
It is known that fatigue resistance and permanent fatigue resistance can generally be improved by increasing the tensile strength and hardness of the spring steel. In the case of a high-strength spring with tensile strength exceeding 1960 MPa, however, types of fracture not observed in conventionally used low-strength steels, such as fatigue failure originating at nonmetallic inclusions and intergranular fracture, occur with high frequency. In the case of a spring subjected to cold forming, moreover, the workability (spring fabricability) of the oil-tempered wire used as the material becomes an important factor. Specifically, when an oil-tempered wire is used to fabricate a coil spring by cold forming, breakage occurs during coiling if the tensile strength of the oil-tempered wire is high, because of the low fracture strain (fracture strain means xe2x80x9cbrittlenessxe2x80x9d of the steel in the present invention).
JP-A-4-247824 teaches that warm coiling is effective for achieving both high strength and good coiling property. This method has, however, been found to be disadvantageous in productivity and operability compared with the commonly used cold coiling method. JP-A-3-162550 asserts that breakage can be prevented by taking advantage of residual austenite to relieve strain by the working-induced transformation that occurs during coiling. The effect of the amount of residual austenite is, however, not certain. While the elongation value is found to increase with increasing amount of residual austenite in a tensile test, the bending angle of a notched test sample in a bending test is found to be unaffected by, or even to increase with, increasing amount of residual austenite.
An object of the present invention is to provide a steel wire for high-strength springs that has both high strength and good spring fabricability. Another object of the present invention is to provide a method of producing the steel wire.
A high strength of 1960 MPa or greater is indispensable for improving spring fatigue strength and permanent fatigue resistance. One particular technique frequently adopted recently for obtaining high strength is a nitriding treatment. A high nitriding temperature of 380-580xc2x0 C. is used. In such cases, the temper-softening resistance is usually increased by adding V, Mo, Ti, Nb or the like in addition to conventional C and Si. Such elements are also added in the present invention.
Even in a simple fatigue test, a spring fabricated of a strength-enhanced steel whose tensile strength exceeds 1960 MPa fractures through a different fracture mechanism from that of conventional steels. Characteristically, compared with conventional steels, fractures more often originate from smaller nonmetallic inclusions or occur as intergranular fractures. It is therefore important to reduce the size of nonmetallic inclusions that can become fracture starting points, to clean the grain boundaries so as to increase intergranular strength, and, particularly, to lower the content of P and S, which are elements that reduce intergranular strength by segregating at the grain boundaries.
Addition of the aforesaid alloying elements to obtain high-strength usually degrades spring fabricability to an unacceptable level. The oil-tempered wire widely used as the material for high-strength cold-formed springs is produced by strand processing in which steel drawn into wire is continuously quenched and tempered. This strand processing is characterized by enabling quenching/tempering to be carried out efficiently in a very short heat-treatment period but tends to result in undissolved carbonitrides remaining in the matrix because the heating period for putting the alloying elements into solid solution is shorter than the heat-treatment period of a hot-formed spring. As these carbonitrides become nuclei for crystal grain formation during recrystallization, they refine the crystal grain boundaries and increase the yield strength. As the yield point rises, the fracture strain falls and notch sensitivity increases. This led to the discovery that cold spring fabricability can therefore be improved by reducing the amount of carbonitrides not entering solid solution. For this it is effective to lower the yield point while controlling carbonitrides during the actual heat-treatment period.
Addition of the aforesaid alloying elements also usually causes residual austenite to remain at the segregation sites and in the vicinity of the old austenite grain boundaries. In some cases residual austenite enhances ductility by relieving strain energy through working-induced transformation but it generally degrades workability in actual cold coiling. Although residual austenite becomes martensite through working-induced transformation, induced transformation occurring during spring fabrication produces local sites of high hardness in the steel. When bruises and scratches arising during handling or other such unavoidable small surface flaws occur, the vicinities of the flaws transform into high-hardness martensite portions and cause extreme local brittleness. It was discovered that these local points of high hardness degrade coiling performance by becoming defects that lead to breakage during spring coiling. In cold coiling of high-strength steel, it is therefore effective to improve workability by minimizing residual austenite and suppressing generation of working-induced martensite.
Therefore, in one of its aspects, the present invention provides:
(1) A steel wire for high-strength springs comprising as steel components, in weight percent,
the balance being Fe and unavoidable impurities, the steel wire having no nonmetallic inclusions of a size greater than 15 xcexcm, a tensile strength of not less than 1960 MPa, and a yield ratio ("sgr"0.2/"sgr"B) of not less than 0.8 and not greater than 0.9 or a yield ratio ("sgr"0.2/"sgr"B) of not less than 0.8 and an amount of residual austenite of not greater than 6%.
In another of its aspects, the present invention provides:
(2) A steel wire for high-strength springs comprising as steel components, in weight percent,
the balance being Fe and unavoidable impurities, the steel wire having no nonmetallic inclusions of a size greater than 15 xcexcm, a tensile strength of not less than 1960 MPa, and a yield ratio ("sgr"0.2/"sgr"B) of not less than 0.8 and not greater than 0.9 or a yield ratio ("sgr"0.2/"sgr"B) of not less than 0.8 and an amount of residual austenite of not greater than 6%.
In another of its aspects, the present invention provides:
(3) A steel wire for high-strength springs according to (1) or (2) above comprising in addition to the defined chemical components, in weight percent,
Ni: 0.1-2.0%,
the steel wire having no nonmetallic inclusions of a size greater than 15 xcexcm, a tensile strength of not less than 1960 MPa, and a yield ratio ("sgr"0.2/"sgr"B) of not less than 0.8 and not greater than 0.9 or a yield ratio ("sgr"0.2/"sgr"B) of not less than 0.8 and an amount of residual austenite of not greater than 6%.
In another of its aspects, the present invention provides:
(4) A steel wire for high-strength springs according to any of (1) to (3) above comprising in addition to the defined chemical components, in weight percent,
Mo: 0.1-2.0%,
the steel wire having no nonmetallic inclusions of a size greater than 15 xcexcm, a tensile strength of not less than 1960 MPa, and a yield ratio ("sgr"0.2/"sgr"B) of not less than 0.8 and not greater than 0.9 or a yield ratio ("sgr"0.2/"sgr"B) of not less than 0.8 and an amount of residual austenite of not greater than 6%.
In another of its aspects, the present invention provides:
(5) A steel wire for high-strength springs according to any of (1) to (4) above comprising in addition to the defined chemical components, in weight percent,
the steel wire having no nonmetallic inclusions of a size greater than 15 xcexcm, a tensile strength of not less than 1960 MPa, and a yield ratio ("sgr"0.2/"sgr"B) of not less than 0.8 and not greater than 0.9 or a yield ratio ("sgr"0.2/"sgr"B) of not less than 0.8 and an amount of residual austenite of not greater than 6%.
In another of its aspects, the present invention provides:
(6) A method of producing a steel wire for high-strength springs comprising a step of subjecting a steel of the chemical composition defined in any of (1) to (5) above to heat treatment at a heating temperature of not less than 920xc2x0 C. and a temperature of the wire at quenching of not greater than 45xc2x0 C. to obtain a steel wire having no nonmetallic inclusions of a size greater than 15 xcexcm, a tensile strength of not less than 1960 MPa, and a yield ratio ("sgr"0.2/"sgr"B) of not less than 0.8 and not greater than 0.9 or a yield ratio ("sgr"0.2/"sgr"B) of not less than 0.8 and an amount of residual austenite of not greater than 6%.